The present disclosure relates generally to polyolefin production and, more specifically, to techniques and systems that employ two or more polymerization reactors in a polyolefin reactor system.
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various industrial products.
Polyolefins may be produced from various monomers, such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks. If one monomer is used for polymerization, the polymer is referred to as a homopolymer, while incorporation of different monomers creates a copolymer or terpolymer, and so on. Monomers may be added to a polymerization reactor, such as a liquid-phase reactor or a gas-phase reactor, where they are converted to polymers. In the liquid-phase reactor, an inert hydrocarbon, such as isobutane, propane, n-pentane, i-pentane, neopentane, and/or n-hexane, may be utilized as a diluent to carry the contents of the reactor. A catalyst may also be added to the reactor to facilitate the polymerization reaction. An example of such a catalyst is a chromium oxide containing hexavalent chromium on a silica support. Unlike the monomers, catalysts are generally not consumed in the polymerization reaction.
As polymer chains develop during polymerization, solid particles known as “fluff” or “flake” or “powder” are produced. The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), copolymer content, comonomer content, modulus, and crystallinity. Different fluff properties may be desirable depending on the application to which the polyolefin fluff or subsequently pelletized fluff is to be applied. Control of the reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, catalyst type, and so forth, may affect the fluff properties.
In some circumstances, in order to achieve certain desired polymer characteristics, the overall polymerization conditions may require that more than one reactor be employed, with each reactor having its own set of conditions. Such polymers may be multimodal polymers, where at least two polymers, each having a different molecular weight fraction, are combined into one polymer product. In a general sense, a polyolefin produced in each reactor will be suspended in a diluent to form a product slurry. The reactors may be connected in series, such that the product slurry from one reactor may be transferred to a subsequent reactor, and so forth, until a polymer is produced with the desired set of characteristics. For example, a bimodal polymer may be produced by two reactors in series; a trimodal polymer may need three, and so on.
In some instances, the flow of slurry that is transferred from one reactor to the next may be unstable (e.g., a non-uniform distribution of solids throughout the slurry), resulting in “salting out” of solids from the diluent. Such a situation may cause clogging during transfer, or may cause a reactor to clog, resulting in reactor fouling. To the extent that clogging may result in deviations from a set of desired reaction conditions, the polymer product produced within a reactor may not meet the desired specifications; that is, the product may be “off-spec.” As may be appreciated, the fouling of one or more reactors within a series may cause the ultimate polyolefin that is produced by the system to be significantly off-spec. In extreme or runaway fouling situations, control of the process may be lost entirely, and a portion of the system employing the reactors in series may become plugged with polymer, requiring significant downtime (e.g., one to three weeks) to clear. Unfortunately, during this time, the polymerization system may not be operated and polyolefin may not be produced. Thus, it may be desirable to avoid fouling by preventing reactor clogging and maintaining stable slurries during transfer. Streamlining such a process employing multiple reactors in series may result in increased efficiency, less system downtime, and increased overall product capacity.